Wireless connectivity, such as the use of wireless local area networks (WLANs) standardized under the IEEE 802.11 standards, is growing more and more widespread. A benefit with wireless connectivity is that fewer cables are required. However, on the other hand, terminals connected to the same access point (AP) share a common medium (radio spectrum) for signal transmission, which can introduce interference and collisions between terminals that need to be handled in some way.
Carrier-sense multiple access (CSMA) is a means to share a channel between terminals without the need for centralized control or strict timing. One flavor of CSMA is CSMA with collision avoidance (CSMA/CA). In CSMA/CA, the channel is sensed before transmission, and in case the channel is busy, the transmission is deferred. CSMA/CA has many desirable properties, one being that it scales relatively well in that the supported data for an individual terminal degrades softly when the number of terminals is increased. This is in contrast to systems where transmission resources are reserved to the individual terminals and for which there therefore might be a distinct limit on the number of connected terminals that can be supported.
In systems based on CSMA, the so-called hidden node problem refers to the situation that one terminal (or “node”) might not hear another node and thus, when listening for a busy medium, might not hear that another node is already transmitting and therefore initiates a transmission that causes a collision.
In order to address this problem, a request-to-send (RTS)/clear-to-send (CTS) signaling scheme can be employed. Such RTS/CTS signaling is defined in Section 9.3.1 of IEEE Standard for Information technology Telecommunications and information exchange between systems Local and metropolitan area networks Specific requirements: Part 11: Wireless LAN Medium Access Control (MAC) and Physical Layer (PHY) Specifications, March 2012 (IEEE Std 802.11—2012). A terminal (in the following referred to as the “source terminal”) which intends to transmit a packet to an AP (or other destination, but the AP is the assumed destination in this discussion) can first send an RTS message to the AP. If the AP receives this request, it can reply with a CTS message to the source terminal, after which the transmission of the actual data from the source terminal to the AP is performed. Consider the case where the source terminal is hidden for another terminal connected to the AP, i.e. the other terminal cannot hear transmissions from the source terminal. However, the other terminal can hear the CTS message from the AP (it can reasonably be assumed that all terminals connected to the AP can hear the transmissions from the AP). Thus, the other terminal knows that a transmission is to be expected from the source terminal (even if it cannot hear the source terminal itself), and thus defers its own transmission to avoid collision with the transmission from the source terminal. There is still a risk that transmissions from the other terminal collide with the RTS message from the source terminal. However, the RTS message is significantly shorter (in time) than a typical data packet, and therefore the probability for a collision with the RTS message is much smaller than the probability for a collision with the actual data packet (had RTS/CTS signaling not been used).
A problem with the RTS/CTS signaling scheme is that it introduces a signaling overhead that might severely limit the overall system performance of a WLAN network.
Use of RTS/CTS for mitigation of the hidden node problem is discussed in U.S. Pat. No. 7,801,096 B2.